0 Replies Latest reply on Mar 1, 2011 6:17 PM by content.librarian

    Why is it that the Voh and Vol specification for QDRII/II+ SRAMs have the same range in the DC specs of Datasheet?

    content.librarian
              
                                                                                                                                                                         
                               
       

                                                                                                                         
       

       

       

      The Voh and Vol specs have a range of (Vddq/2-0.12) V to (Vddq/2+0.12) V

      This is defined by the QDR consortium.

      Basically, for an impedance variation from 35 ohms to 70 ohms, there is a corresponding current Ioh and Iol. Corresponding to a specific Ioh and Iol current the Voh and Vol values will be different.
      Hence the receiver can make out the difference between Voh and Vol for a specific impedance setting.

      The DC test specifications for Voh and Vol which are specified by Notes under the DC Electrical Characteristics section of the datasheet are actually for calculating the Ioh and Iol level of the drivers for a Voh and Vol of Vddq/2. The reason +/-0.12 is added is to provide the voltage variation for the Voh and Vol based on the +/-15% accuracy of the output impedance circuitry. For this +/-15% accuracy you will see a voltage variation of +/-0.12 on the outputs.

      The IOH and IOL values can be calculated as below:

      |IOH|=(VDDQ/2)/(RQ/5)+/-15% for 175Ω ≤ RQ ≤ 350Ω.
      |IOL|=(VDDQ/2)/(RQ/5)+/-15% for 175Ω ≤ RQ ≤ 350Ω.

      For eg: for a VDDQ=1.5V and RQ=250Ω

      |IOH|=(0.75/50)= 15mA
      |IOL|=(0.75/50)= 15mA

      If we consider the -15% accuracy for 50ohm you are looking at 42.5ohm output

      |IOL|=(0.75/42.5)= 17.4mA

      The difference between the nominal and variation is 2.4mA

      Vol variation = 0.0024*50 = 0.12V.

      The same explanation holds true for Voh.